Priority is claimed herein with respect to Application No. 198 39 895.6 filed in the German Patent Office on Sep. 2, 1998, the disclosure of which is incorporated herein by reference.
The invention relates to a method of receiving and superpositioning radio signals having identical contents and different frequencies.
Telecommunications systems for decoding, equalizing and receiving radio signals, such as are already realized in numerous modular units in, for example, Viterbi decoders, RAKE receivers and lattice equalizers, edit the data stream. The European Patent publication EP 0 492 419 B1 discloses a Viterbi decoder system having an equalizer with variable properties. The equalizer includes a transversal filter structure and time-delay members for a time unit delay by 1 bit. A weighting of the partial signals of the data stream read into the equalizer allows the wave shape of the input signal to be modified and equalized. The signal components are re-added prior to an A/D conversion for further digital processing.
In the known solutions, however, frequency shifts due to the Doppler effect are not detected and corrected completely, or at all.
It is the object of the invention to provide a method with which the receiving capability, and particularly the receiving capability of moving objects, is increased.
The above object generally is achieved according to the present invention by a method of receiving data signals having identical contents and different frequencies, wherein: the frequency position of the data signal is analyzed by use of an FFT; this analysis is used to control local oscillators for mixing the input frequencies to IF signals such that, through separate mixing of the data-signal components, the individual partial waves are frequency-corrected to a common IF mean frequency; and thereafter the frequency corrected individual partial waves are superpositioned. Further advantageous embodiments and modifications of the invention are disclosed.
The crux of the invention is the superpositioning of received signals having identical contents and different frequencies. The difference in frequency can be caused by different frequencies when the signal is being transmitted by one or more transmitters, or by a Doppler shift of a plurality of partial waves due to a moving receiver.
The invention includes a method in which the frequency position of the data signal is analyzed by means of an FFT, and this analysis is used to control local oscillators for mixing the input frequencies to IF signals. Through separate mixing of the data-signal components, the frequencies of the local oscillators are selected such that the individual partial waves are frequency-corrected to a common and unified IF mean frequency, and these partial waves are then superpositioned. The spectrum of the received signal, which is determined by the FFT, defines the frequencies of the local oscillators.
The mixing to IF signals through separate mixing stages is effected in analog form prior to an A/D conversion, or digitally after an A/D conversion.
Digital mixing is primarily suitable for correcting small frequency shifts, such as those that occur in a Doppler shift. For each branch, different signal transit times of the partial waves can be matched to one another and compensated by time-delay members.
An FFT analysis of different frequencies also involves a transmission of the signal by one or more transmitters. Doppler-shifted partial waves are also analyzed in the FFT analysis, with the position relative to the transmitter and the inherent speed of the receiving object being utilized in the analysis.
In a superpositioning of the frequency-corrected partial waves, which are consequently at the same IF mean frequency, a weighting related to the respective signal level is effected.
An advantage of the invention is that the overall receiving capability can be increased through a reception from a plurality of transmitters.
The improvement in the receiving quality is especially advantageous in moving receivers. Here, frequency shifts that are stipulated by a Doppler shift are completely corrected.